A solar cell technology is to directly convert light, that is, solar energy into electric energy by utilizing a photovoltaic effect, and most commercialized solar cells are inorganic solar cells using inorganic materials such as silicon. However, the inorganic solar cells have disadvantages in that manufacturing cost is increased due to a complicated manufacturing process, and high-priced materials are required, and accordingly, research into a dye-sensitized solar cell and an organic solar cell manufactured with low cost through a relatively simple manufacturing process and with low-priced materials has been actively conducted.
In general, the dye-sensitized solar cell consists of two electrodes (a photo electrode and a counter electrode), semiconductor nanoparticles (mainly titanium dioxide), dye, and a liquid electrolyte. When solar light (visible light) is absorbed onto an n-type nanoparticle semiconductor oxide electrode having a surface onto which dye molecules are chemically adsorbed, the dye molecules form electron-hole pairs, wherein the electrons are injected into a conduction band of semiconductor oxide and are transferred to a transparent conductive film through interfaces between nanoparticles, thereby generating a current, and the holes receive the electrons by an oxidation-reduction electrolyte and are reduced again. The dye-sensitized solar cell is operated by the electron circulation mechanism as described above. The dye-sensitized solar cell using high-priced ruthenium-based complex as a dye according to the related art has received academic attention due to an energy conversion efficiency of over 10%, but has difficulty in commercialization due to a problem that long-term stability of a device is deteriorated.
Regarding this, a liquid electrolyte component part is very closely related to the long-term stability of the device. A volatile electrolyte solution in a solution state containing iodine has an excellent advantage in view of an energy conversion efficiency, but also has a disadvantage that if the electrolyte leaks or becomes volatile during use time, a fatal problem in stability of the device may occur. In particular, an iodine component of the electrolyte solution may cause chemical decomposition of the dye molecules, and may seriously destroy a module grid of the metal component due to an action between a small amount of oxygen and moisture when the cell is operated for a long time.
As a method for solving the problem of the dye-sensitized solar cell by the liquid electrolyte, The Gratzel Group in Switzerland reported in 2013 that 7.2% energy conversion efficiency could be obtained by using Y123 organic dye, and substituting the existing liquid electrolyte with a solid type hole conductive organic material, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (Spiro-OMeTAD), which had the highest value of efficiency of the solid type dye-sensitized solar cell reported to date, but it was lower than an energy conversion efficiency of the existing dye-sensitized solar cell using the liquid electrolyte solution.
Meanwhile, the organic solar cell is composed of a first electrode, a photoactive layer consisting of an electron donor (D) and an electron acceptor (A), and a second electrode, and may further include an electron transport layer and a hole transport layer. When light is projected to the organic solar cell, positive charges (holes) and negative charges (electrons) are produced in the photoactive layer, the electrons are moved to an electrode of an upper part of the photoactive layer, and the holes are moved to the hole transport layer. The hole transport layer of the organic solar cell manufactured by using a mixture of poly(3,4-ethylenedioxy-thiophene) (PEDOT) and poly(styrenesulfonate) (PSS) according to the related art causes corrosion of a lower electrode layer consisting of a metal such as ITO due to high acidity of PEDOT:PSS, and as a result, lifespan of the hole transport layer is reduced, which is recognized as a problem in commercialization of the organic solar cell.
Accordingly, it needs to design and develop a novel hole transporting material of an organic material-based small molecule capable of substituting the existing hole transporting material, PEDOT:PSS, in order to improve the power conversion efficiency and the lifespan of the device in the organic solar cell.
Meanwhile, the perovskite solar cell has received attention as an important device due to excellent photovoltaic properties, cost reduction, and easy processes. The perovskite solar cell without the hole transporting material had a low charge extraction and charge recombination at an interface as compared to the perovskite solar cell including the hole transporting material, resulting in a drop of an open-circuit voltage and a charging rate. Therefore, in order to have higher power conversion efficiency (PCE), it is required to increase the charge extraction and to mitigate unwanted charge recombination at the interface. To this end, a role of the hole transporting material (HTM) is important in the perovskite solar cell.
A research into a technology of better improving the power conversion efficiency which is one of the main characteristics of the perovskite solar cell has been conducted. Recently, a power conversion efficiency of the perovskite solar cell using spiro-OMeTAD as the hole transporting material was achieved to be 20%. However, the spiro-OMeTAD has a complicated synthesis, a high price, and a low carrier mobility of charges, which may be limited in commercialization of the perovskite solar cell. A hole transporting material based on a polymer has been also widely used, but has problems in that stability of a device may be deteriorated due to acid environment, and there is difficulty in reproducibly and uniformly controlling factors such as a molecular weight of the polymer, polydispersity, and stereoregularity that directly affect performances of the solar cell, synthesis or purification process is complicated, and charge mobility is low.
In general, dopants such as Li-TFSI, t-BP, etc., are added to improve the power conversion efficiency in the perovskite solar cell. However, it is required to design and develop a novel hole transporting material of the organic material-based small molecule which is effective without including the dopants in order to secure device stability of the perovskite solar cell.